1. Field of the Invention
The present invention relates generally to a light source device including a discharge tube, a discharge medium sealed inside the discharge tube, and electrodes for exciting the discharge medium, and also relates to an image reader employing the light source device.
2. Related Background Art
The development of discharge lamp devices in each of which a discharge tube and electrodes arranged inside and/or outside the discharge tube are provided has been promoted. Discharge lamps in each of which an inner electrode arranged inside a discharge tube and an outer electrode formed on an outer surface of the discharge tube are provided with view to stabilizing the discharge state and improving the light emission efficiency are used widely. These discharge lamps are caused to emit light by applying a voltage across the inner electrode and the outer electrode.
As such a discharge lamp, a discharge lamp device disclosed in JP6(1994)-163005A is well known. FIG. 14 schematically illustrates the conventional discharge lamp device. The conventional discharge lamp device 900 includes an inner electrode 901, an outer electrode 902, and a discharge tube 903. Inside the discharge tube 903, a rare gas is sealed. The inner electrode 901 is arranged inside the discharge tube 903, and the outer electrode 903, which is in a linear form, is arranged on an outer surface of the discharge tube 903. The inner and outer electrodes 901 and 902 are connected with a driving circuit 904. The application of a voltage to these electrodes by the driving circuit 904 causes the discharge lamp device 900 to emit light.
JP 10(1998)-112290A discloses a discharge lamp device having a spiral outer electrode formed on an outer surface of a discharge tube.
To obtain light emission with high brightness in a discharge lamp device, it is effective to raise the gas pressure inside a discharge tube or to increase an input voltage. However, the foregoing conventional discharge lamp devices have a problem that a rise of the gas pressure or an increase in the input voltage tends to cause constricted discharge. In the case where the discharge is constricted, it is impossible to obtain a brightness commensurate with input power, decreasing the light emission efficiency. Furthermore, in the case where the discharge is constricted, the problem of a temperature rise in the tube wall of the discharge tube occurs.
The causes of the constriction of the discharge are described below. FIGS. 15A, 16A, and 17A are views schematically illustrating discharge states. FIG. 15B is a schematic cross-sectional view taken along a line XIVB—XIVB in FIG. 15A. FIG. 16B is a schematic cross-sectional view taken along a line XVB—XVB in FIG. 16A. FIG. 17B is a schematic cross-sectional view taken along a line XVIB—XVIB in FIG. 17A.
In the case where the gas pressure inside the discharge tube 903 is not more than 1 kPa, upon input of power to the discharge lamp device 900, discharge starts at a portion where the inner and outer electrodes 901 and 902 are in the closest proximity with each other. In the foregoing portion, constricted discharge occurs once, but since a discharge substance inside the discharge tube 903 is present in a low amount and hence the mean free path of electrons is sufficiently long, the discharge path tends to expand. As a result, as shown in FIGS. 15A and 15B, diffused discharge occurs with a central portion of the discharge tube 903 as the center of diffusion. In the diffused discharge state, the discharge medium can be excited efficiently in a wide region inside the discharge tube 903, thereby improving the excitation efficiency, and increasing the light output relative to the input power.
As the gas pressure inside the discharge tube 903 rises above 1 kPa, the discharge substance inside the discharge tube 903 increases, thereby shortening the mean free path of electrons. Therefore, the discharge gradually is constricted to one point so that the discharge is maintained. Besides, as the gas pressure rises, the resistance in a portion in the vicinity of the outer electrode 902 becomes lower than a resistance in the central portion of the discharge tube 903. This causes the discharge to be constricted along the outer electrode 902, as shown in FIGS. 16A and 16B. Here, since the constricted discharge excites only a part of the discharge medium, the excitation efficiency decreases, thereby decreasing the light output. Furthermore, since energy not used in the excitation, which is regarded as energy loss, is radiated mainly in the form of heat, this causes the temperature of the discharge tube to rise. In this state, though the light output is stable, the light output relative to the input power is saturated, thereby increasing heat generation.
Furthermore, as the gas pressure is raised further, discharge occurs selectively in a portion in the vicinity of the outer electrode 902 and having a low gas concentration. Therefore, as shown in FIGS. 17A and 17B, the discharge meanders and is destabilized. As a result, the light amount of the discharge lamp device 900 is destabilized, which makes it difficult to obtain a brightness commensurate with the input power.